From the past, plasma spraying and detonation spraying techniques have been widely utilized for forming dense spray coatings on surfaces of ceramic or a metal. In these thermal spraying techniques, metal, metal oxide and the like are often used as the spray powder.
More particularly, it is a common practice in the art to thermally spray metal or metal oxide particles onto metal or ceramic substrates to form a coating thereon for imparting heat resistance, abrasion resistance and corrosion resistance. The properties of the coating thus formed depend on the spraying conditions as well as the properties of powder particles to be sprayed.
The particle powders suitable for thermal spray coatings typically include (1) a fused and ground powder obtained by melting a starting material in an electric furnace, cooling the melt for solidification, and pulverizing the solid in a grinding machine into particles, followed by classification for particle size adjustment; (2) a sintered and ground powder obtained by firing a raw material, and pulverizing the sintered material in a grinding machine into particles, followed by classification for particle size adjustment; and (3) a granulated powder obtained by adding a raw material powder to an organic binder to form a slurry, atomizing the slurry through a spray drying granulator, and firing the particles, optionally followed by classification for particle size adjustment. The starting material used in the preparation of these powders (1) to (3) is selected as appropriate and has been developed in accordance with the cost and the desired properties of the end spray coating.
While plasma processes are involved in the recent semiconductor fabrication art, rare earth-containing compounds have been developed as a wafer processing component in corrosive halide gases because they have high resistance to plasma.
When spray coatings are applied to components in semiconductor manufacturing apparatus, the spray coatings are required (a) to contain less impurity elements other than the predominant constituents and (b) to have a less irregular, smooth surface bearing less fines, which means to suppress dusting during wafer processing. To meet these requirements, it becomes crucial how to control the properties of powder particles to be sprayed as well as the spraying conditions.
The thermal spray particles have to meet the requirements that (i) they can be consistently fed without disintegration at a quantitative rate to the plasma or flame during spraying, (ii) they are fully melted during spraying (in plasma or flame) and (iii) they are highly pure. These requirements are quantitatively expressed by more than ten physical parameters and elemental analysis data of particles.
Since the thermal spray particles are fed to the spray gun through a narrow flowpath such as a transportation tube, whether they can be consistently fed at a quantitative rate is largely affected by the flow thereof among other physical parameters. However, the fused or sintered and ground powder resulting from method (1) or (2) has irregular shapes which lead to the drawback that the sprayed coating has large irregularities. Additionally, the fused and ground powder has the other drawback that the content of impurities other than the constituent elements is high, and the sintered and ground powder has the other drawback that impurities are often introduced in the grinding step.
Developed as a solution to these problems of the ground powders was the granulated powder obtained by method (3), that is, having the advantage of smooth flow due to the spherical or nearly spherical shape of particles. An additional advantage of the granulated powder is that a relatively pure granulated powder can be readily prepared by reducing impurities in the starting material.
However, a starting powder of a certain type can give rise to the problems that particles of a shape dissimilar to sphere are granulated therefrom and that the starting powder sticks to granulated particles. An additional problem is that a degradation of flow is incurred particularly when particles have a small diameter.
When a coating is sprayed using the granulated powder, the fines which have not been granulated (i.e., have been carried from the starting powder) are incorporated in the coating or stick to the surface of the coating. This causes a substantial amount of dust to generate when the coated component is used in semiconductor equipment or the like.
In order that particles of metal compound be thermally sprayed, without dust generation, to form a coating having improved bond strength, the particles must be completely melted in the flame or plasma during the spraying step and the supply of the feed particles must be precisely controlled. Particularly when particles of rare earth-containing compounds are used for thermal spraying, because of their high melting point, they should preferably have a smaller average particle size so that they may be completely melted.
In the event where granulated powder is prepared using a spray drying granulator, however, it is difficult to selectively prepare a fraction of particles having a small average particle size. Inevitably, particles having a relatively large average particle size are concomitantly produced. Such particles with a large average particle size have a large weight and are not completely melted when fed into the plasma flame so that they are incorporated in the sprayed coating as unmelted particles, which become one cause of incurring irregularities in the coating.
One approach for overcoming the above-mentioned problems is to reduce the particle size of the starting material for eventually holding down coating surface irregularities. This approach is undesirably accompanied by generation of fines, a degradation of flow and a difficulty of precise metering. As a result, surface irregularities develop and the coating becomes less dense. Furthermore, the fines which stick to particle surfaces without being granulated are not introduced into the plasma flame during the spraying step and are thus kept unmelted so that they are incorporated in or stick to the sprayed coating.